1. Field of the Invention
The present invention relates to a device for separating material web sections from a moving endless material web. In particular, this invention relates to a device for separating material web sections in conformity with print marks applied to the web for separating letter envelope cuts from a printed paper web.
2. The Prior Art
In the manufacture of letter envelopes from a continuous web, individual cuts are formed by a rotating knife (or cutter) roll, which cooperates with a counter roll or counter bar. The paper web is pulled from a supply roll of paper by take-off or feed rolls and pushed between the knife roll and the counter roll. After the separation cut has been made, the cut letter envelope is received by transporting means, and fed to further processing operations. The length of the letter envelope cuts is determined by the length of the material web pushed by the feed rolls between two cutting operations into the roll gap between the knife roll and the counter roll. It is common practice to operate the knife or cutter roll as a so-called cycled roll, i.e., to have it rotating with the speed of the machine and to obtain or effect a change in length exclusively by changing the raze of revolution and thus by changing the rotational steed of the feed rolls transporting the material web.
This was accomplished in the early stages of letter envelope machine engineering by exchangeable toothed gears in the feed roll drive. This means that only stepped changes in length were possible.
Because of this problem, infinitely variable speed transmissions and differential drives were employed later instead. Processor-controlled individual drives have been increasingly used in more recent years in machine engineering. These drives provide the opportunity to change the relative speed between feed rolls and the knife or cutter roll within wide limits and without additional expenditure. However, problems arise if a highly defined cutting position must correspond with a mark applied to the web. This is normally the case in connection with preprinted webs, where the cut must conform to the print pattern. The print pattern itself may serve as the mark.
However, a special register mark is usually printed on the web for marking the spot or point through which the cut has to extend.
The length of the cuts is therefore preset by the spacing of the marks relative to each other.
This makes manual adjustments of the line of the cut extremely difficult, because in addition to the length of the section, the phase position of the cut relative to the mark must be accounted for. This is complicated further by the fact that after the necessary adjustments have been made, even the most minor remaining or future feed errors later can lead to visible summation errors and require additional corrective intervention.
Therefore, the course or line of the cut must be constantly monitored by the operating personnel.
A system is known from U.S. Pat. No. 5,241,884 which discloses that after the length of the cut has been basically adjusted by hand, the system automatically sets itself to a correct course or line of the cut via register marks applied to the material web.
The cutting roll is rigidly coupled to the main drive train of the machine, whereas the feed rolls are equipped with their independent drive in the form of a servo-motor.
Sensors are arranged both on the knife or cutting roll and the pair of feed rolls. These sensors are connected to a processor in the machine control unit. The sensor on the knife roll constantly detects the position of the knife or cutter and the sensor on the pair of feed rolls detects the rate of revolutions or rotational speed of the pair of feed rolls. Furthermore, the length of the cuts, which is determined by measuring the spacing between the marks, is manually entered via a keyboard. Based on these values, the processor computes the required relative speed between the pair of feed rolls and the knife roll or cutter and adjusts the feed roll drive accordingly. This leads to cuts having the length corresponding to the entered spacing of the marks. What remains to be adjusted, provided that the length input is correct, is the phase position of the cut relative to the mark. For this purpose, an optical sensor is arranged on the material web, which responds to the register marks and reports or signals the detection of such a mark to the processor. Based on the knowledge of the instantaneous position of the mark and the knife and the moving speeds, the processor determines where the knife or cutter will cut or separate the material web relative to the register mark. If the processor finds that the cut and the mark do not coincide, the rate of revolutions or rotational speed of the feed rolls is changed to reduce the errors until the desired course or line of the cut is obtained.
The drawback of this solution is that to start up the cutting device, it is first necessary to determine the mark spacing in order to enter this spacing by hand.